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New device sniffs out terrorists' bombs

A super-sensitive electronic nose that can detect explosives may make physical searches a thing of the past. James Urquhart reports.

The new test detects tiny concentrations of the vapours given off by explosives, more than current swab tests can achieve.

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Terrorist bomb plots are an ever-present security threat and detecting them is becoming harder. The latest news, of a possible non-metallic bomb that can evade scanners, is not just alarming but will ensure even longer airport security queues. A new device, however, that can detect minute traces of explosives just by sniffing the air could, in the not too distant future, provide more security with less intrusive procedures.

Tests for explosives currently involve passengers and their luggage being swabbed to collect samples which are tested for known explosive substances. The procedure is cumbersome and error-prone – not everyone is tested, the instruments lack the sensitivity to detect trace amounts, and there is no single device that can detect all known explosives.

The new device, developed in Israel, could answer all those concerns, detecting any explosives vapours in the air.

The challenge, however, is that some explosives have extremely low volatility – very few molecules evaporate from their surfaces to float free in the air. The volatility of TNT, for example, is only a few parts per billion at room temperature, which means there are only a few molecules of TNT present per billion molecules of air. Other explosives go down to the parts-per-trillion or even parts-per-quadrillion range, making them even harder to detect. But, according to a report in Nature Communications, the new device is up to it.

When explosives vapours come into contact with the device’sreceptor molecules they stick.

“Our system is small, ultrasensitive and selective, and can detect a broad range of explosive species using a single real-time miniature electronic platform,” explains Fernando Patolsky, who helped develop the new device with colleagues at Tel Aviv University. “The sensitivity achieved was really surprising, as we can detect traces of explosives down to the parts-per-quadrillion concentration range.”

Patolsky’s detector is built around an electronic chip packed with 144 electrical nano-sized sensors. Each sensor is made up of silicon nanowires that bristle with one of eight different receptor molecules sensitive to explosives.

When explosives vapours come into contact with the device’s receptor molecules they stick, altering the nanowires’ electrical conductivity. But just as our noses don’t have dedicated olfactory receptors for each smell we might encounter, the device doesn’t have a specific sensor for every flavour of explosive – it’s all about pattern recognition. So when TNT, for example, enters the device, it will bind very tightly to receptor 1, but quite lightly to receptor 2. This gives TNT a unique, characteristic “fingerprint”. Similarly for other explosives, pattern recognition allows the device to achieve very high sensitivity. The developers say the device produces very few false alarms.

The new sensor can detect a wide range of explosives including the nitroglycerin family used in dynamite. – Charles D Winters/Getty Images

In tests, the device could detect the peroxide-based explosive TATP from five metres away, and TNT from up to four metres away.

While no single component of the device is conceptually new – other Israeli researchers are trying a similar “electronic nose” concept to diagnose disease in samples of a patient’s breath, for example (see Sniffing out Cancer), the advance is in combining these components into a high-functioning device, says Michael Strano, who investigates nano-sensors at the Massachusetts Institute of Technology in Boston. “The work combines several important elements to great effect, yielding very high levels of sensitivity and selectivity,” he says. “This approach has been proposed and demonstrated using collections of sensors for several decades but has never proven successful enough for practical use.” This could be the one to finally buck that trend, he says.

Patolsky hopes so. He is working with Israeli based spin-out company Tracense – which part-funded the work and of which Patolsky is chief scientific officer – to develop a series of prototypes to eventually bring the device to market.

“Extensive field tests and regulatory issues are still required before we see these sensors in the market, but I strongly believe this platform will represent the next generation of sensors with enhanced capabilities,” he says.

And perhaps you’ll never have to be swabbed at airport security again.